Optical sensor element

ABSTRACT

An optical sensor element for detecting organic compounds, particularly hydrocarbon. The sensor element permits the selective detection of substances by use of units which produce spectral displacements. This is realized by employing a spectral band filter, which comprises at least one layer of variable optical thickness of λ/4 and λ/2, respectively, or a multiple thereof, and said layer exhibits hydrophobic properties and includes organic affinity groups, the selection of which is adaptable to the substances to be detected.

BACKGROUND OF THE INVENTION

The invention relates to an optical sensor element for detecting organiccompounds, particularly hydrocarbon, which can be utilized, for example,in environmental and health protection.

Hydrocarbon may pollute the environment and may be harmful to humanhealth. Hence, the measurement of hydrocarbon is an important task.Conventional methods for detecting hydrocarbon are chromatography,IR-absorption methods, acousto-optic measuring methods, heatconductivity measurements, heat effect measurements of the catalyticdecomposition of the hydrocarbons, or electrolytic conductivitymeasurements.

The first three measurement methods use expensive equipment and onlypermit application in laboratories. The other methods are suited forembodying measuring feelers which can be installed at different placesin the environment or industrial processes. Their lifetime is short whensubject to corrosive ambiance. Since their function relies on electricalprinciples they are, when used in an explosive environment, onlyapplicable with explosion-protective measures. The measuring andamplifier electronic, respectively, has to be installed in the directvicinity of the measuring feeler. In contrast thereto, fiber-opticalsensors conventionally do not require any additionalexplosion-protective measures and they permit greater distances, up tothe kilometer range, between the measuring location and the signalevaluation (remote sensing) due to the extremely interference-safe andlow attenuated measuring signal transmittance via light cable. Thus, incontrast to electronic sensors, applications become feasible even underadverse environmental conditions and at measuring places difficult toaccess. Recently, solutions of fiber-optical sensors have become knownfor detecting hydrocarbon.

So it has been proposed, for example, to make light cables out of porousglass and, in a next step, to secure a chemical species, which issensitive towards the substances to be detected to the glass surface [M.Tabacco et al: "Chemical Sensors for Environmental monitoring"; SPIEVol. 1587 Chemical, Biochemical and Environmental Fiber Sensors III(1991) 271]. However, a such prepared porous fiber is scarcely suited asa sensor in practical use since it is very fragile. Furthermore, withthis sensor principle, the intensity measurements on light conductingfibers are an ill suited means to ensure a stable and quantitativelyreliable detection of the concentration of the substances to be detectedsince. Any variation of the light intensity of the light source or ofthe fiber feed to the sensor distort the measuring signal.

Other examples use a light conducting fiber which is composed of aquartz glass core and an optical coat made of silicon [J. P. Conzen, etal: Characterization of a Fiber-Optic Evanescent Wave Absorbance Sensorfor Nonpolar Organic Compounds"; Applied Spectroscopy Vol. 47, 6 (1993)753 or C. Ronot, et al: "Detection of chemical vapours with aspecifically coated optical fibre sensor"; Sensors and actuators B, 11(1993) 375-381]. The silicon coat protects the fiber core against waterand other polar substances. Organic compounds, however, such ashydrocarbon compounds, are able to diffuse into the silicon coat.Refractive index variations, swelling and optical absorption variationsmay result. These variations affect the transmission of the lightconducting fiber, since a definite portion of the light conducted in thelight conducting fiber also enters into the optical coat as a so-calledevanescent wave where it is subject to the variations. Sensors basedupon these effects mostly require light conducting fibers of severalmeters to ensure a sufficient sensor sensitivity and, hence, some space,since the evanescent light portion only makes a small part of the entirelight conducted. In this case, it is also true that the principle oftransmission measurement on light conducting fiber sensors isdisadvantageous with respect to the stability and reproducibility of thesensors. Such disadvantages may deteriorate the advantages otherwiseinherent in optical fiber sensors, such as explosion safety, immunity toelectromagnetic interference fields, optimal electric potentialsplitting or bridging of great distances between measuring place and thelocation of the electronic signal evaluation and, thus, may preventpractical use.

It is further known that a safe and non-distorted transmission ofoptical sensor signals is feasible with light conducting fibers, evenover wide distances, when the measuring information is transmitted as aspectrally encoded optical signal wherein the variations of themeasuring information are rendered measurable as spectral displacementsof maxima or minima of the light intensity. An example of such aprinciple is a fiber-optical moisture sensor [EP 0 536 656 A1] andassociated method for signal evaluation [EP 0 538 664 A2]. The propermoisture-sensitive element of the fiber-optical moisture sensor is anoptical narrow-band filter constituted of a stack of sandwiched opticallayers selected from inorganic dielectric material with alternating highand low optical refractive index. The optical layer thickness always isa multiple quarter and a multiple half, respectively, of an adjustablemean working wavelength of the measuring light. It has been known thatsuch layers, when manufactured by vacuum deposition, are porous and, inthe presence of water vapor in the ambiance, can take up water [H. Koch:"Optische Untersuchungen zur Wasserdampfsorption in Aufdampfschichten"phys. stat. sol. 12 (1965) 533-543]. The optical refractive indexes ofthe individual layers vary with the absorption of water and the filterspectrum is displaced towards longer wavelengths.

The spectral displacements of the filter spectra are very preciselymeasurable even over wide light conducting fiber paths and fiber opticalmoisture sensors provided with such sandwich stacks permit a veryreliable evaluation.

However, such layers according to the state of the art are onlysensitive with respect to water vapor, apart from small undesiredtransverse sensitivities of other vapors such as, for example, alcoholicvapor or ammonia.

It is an object of the invention to provide an optical sensor elementfor detecting organic compounds, particularly hydrocarbon, that permitsthe selective detection of the latter by use of units which producespectral displacements.

SUMMARY OF INVENTION

The invention provides at least one optical layer, which is part of aspectral band filter constituted of a plurality of λ/4 and λ/2 layers,respectively, and which possesses defined characteristics with respectto the affinity relative to at least one substance to be detected. Thecharacteristics have the effect that the optical thickness of a layerand a stack of such layers, respectively, is subject to variations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a and FIG. 1b show possible deposits and their formation on porouslayers according to the invention,

FIG. 2 shows application of a stack of layers according to the inventionwhich is used as a sensor head of a fiber optical detector operating inreflection mode,

FIG. 2a is a detailed schematical representation of the stack of layersaccording to FIG. 2, and

FIG. 3 shows a possible application of an extended stack of layersaccording to the invention, which are scanned in transmission by, forexample, a CCD-camera.

DETAILED DESCRIPTION OF THE INVENTION

The starting point for all further considerations is a spectral bandfilter of a plurality of optical thin layers having a thickness of λ/4and λ/2, respectively, or a multiple thereof in which layers arearranged alternatingly high refracting (n_(h)) and low refracting(n_(n)) with respect to the mean working wavelength of the measuringlight utilized. It is of no consequence within the scope of theinvention whether the spectral band filter is embodied as a transmissionfilter or as a reflection filter, an edge-type filter, a narrow-bandfilter, etc. The filter includes at least one layer of variable opticalthickness (which is the product of refractive index and geometricalthickness) of λ/4 and λ/2, respectively, or a multiple thereof and whichexhibits hydrophobic properties and includes organic affinity groups,the selection of which is adaptable to the compounds to be detected, andis exclusively constituted of such layers, respectively.

In a first embodiment, as shown in FIG. 2 and 2a a layer 31 of aspectral filter layer system according to the invention is a dielectricinorganic material, for example, of at least one metal oxide which, dueto its manufacture, is provided with pores P of a maximum size up to 10nm. Such a size of the pores ensures that the optical layer appearsoptically homogeneous and substantially no light scattering occurs. Thelayer is enclosed on both sides by alternating high refracting n_(h) andlow refracting n_(n) thin films in the form of a reflection filterwherein, on the detection side (on the right side of FIG. 2a), thereflection filter layers are also provided with pores which areinterconnected with the pores in the layer 31 to permit the entry ofsubstances to be detected. According to the invention, at least thepores in the layer 31--the surfaces of the pores and also the surface ofthe layer 31 naturally are extremely hydrophilic--are covered bymolecules which render the character of the surface stronglyhydrophobic. Such a deposit is obtained by, for example, silylation.Organosilane compounds, for example, are suited for it, which have thegeneral chemical formula R_(Z) SiX.sub.(4-Z) with (1≦Z≦4). X designatesa hydrolysable group, such as an alkoxy-group, or a halogen, such aschlorine, and R stands for a non-hydrolysable organic radical which isadapted to provide the pore surface with a special functional property,in the sense of the invention, in particular a hydrophobic property.Methoxy-(--OCH₃) and the ethoxy-(OCH₂ CH₃)-group are examples for thealkoxy-groups.

By chemical reactions of such an organosilane compound with a hydratedmetal oxide surface, the hydrolysable groups X react with the hydrogenof the hydroxyl groups on the metal oxide surface and are separated, sothat the silicon with its functional group R is chemically combined withthe metal oxide surface. Feasible surface formations of the kindmentioned are indicated in FIGS. 1a and 1b.

In a simple example, methylchlorine-silane, such as (CH₃)₂ SiCl₂, issubstituted as an organosilane compound. Since methylchlorine-silaneboils between 57° C. and 70° C., the operation preferably takes placeslightly above these temperatures and permits the vapor to react withthe pore surfaces. During operation, a thin methylpoly-siloxan filmdeposits on the interior of the pores, while the liberated hydrogenchloride evaporates.

A surface deposit produced in this manner has a distinctive affinitytowards non-polar and low-polar molecules so that a moistening occurswith such compounds, whereas water is repelled. Since the inventivelycoated pore surfaces are strongly concave, a vapor pressure reduction inthe moistening liquid takes place. The greater the reduction the moredistinct is the moistening power and the smaller are the pores. Hence,the vapor pressure reduction leads to a condensation of vapors in thepores, these vapors originate from a gas mixture which, for example,contains, inter alia, condensable non-polar vapors, such as organicsolvent vapors or other condensable hydrocarbon compounds. The cavitiesof the individual pores which were initially filled with gas and vapor,respectively, are now filled with a liquid. Thus, the optical refractiveindex of the porous layer increases and, hence, its optical thickness.

In order to render the variations of the optical thickness of the layermeasurable, the latter is so adjusted in the course of manufacture ofthe layer that it equals a quarter or a half of a mean workingwavelength of a selected light spectrum or an integral multiple thereof.According to optical relations, known to one skilled in the art, opticallayers show transmission or reflection maxima at these wavelengths. Theoriginal mean working wavelength is displaced proportionally to theoptical variation of thickness when an inventive variation of theoptical thickness takes place due to condensation of non-polar vapors inthe pores. It is feasible to evaluate such spectral displacements withhigh precision and reliability by utilizing spectral measuringtechniques as disclosed in EP 0 538 664 A2.

In order to set the transmission maxima or minima favorable forevaluation to obtain a still higher contrast, it is advantageous toprovide the porous layer and the stack of layers, respectively, on bothsides with semi-transmissive reflecting areas. According to theinvention, the layers are so embodied that at least the reflecting areason one side are also porous and all pores of the mean porous sensitivelayer, which substantially determines the working wavelength, haveaccess to the outside through which the vapor molecules may enter thepores from ambiance and vice versa. It is feasible to make thereflecting areas of dielectric matter the optical thickness of whichcorresponds to a quarter of the light wavelength or of partiallytransmissive metallic matter.

In a further embodiment of the inventive sensor element it is feasibleto non-displaceably connect the spectral filter layer system 3 via thesensing head 2 to the end faces of light conducting fibers 1, as shownin FIG. 2a, or directly attach them to the former. The light conductingfibers 1 are employed to feed the illumination I [λ] into the layersystem 3 and to feed back the filtered light I [λ(C)] to an opticalevaluation unit indicated in FIG. 2 by a dashed-line frame, wherein C isthe concentration of the substances to be detected. It also lies withinthe scope of the invention to embody the spectral filter layer system 3,31 as an extended spatial plate attached to a transparent mount 4 (asshown in FIG. 3) and to carry out the spectral evaluation at a remoteplace, for example, via a CCD-camera 5.

It lies, of course, within the scope of the invention to use others thanthe described organosilane compounds in order to either increase thetemperature stability of the siloxane films or to vary the polarcharacteristic or to enhance the selectivity of the interaction withspecial molecules.

Particularly, the dielectric inorganic basic matter for the porous layercan be replaced by organic matter, the molecules of which form cage-likecavities, such as cyclodextrin, wherein the selectivity with respect tocertain molecules is obtainable by the conformity of the size of saidmolecules with the size of the molecular cage.

Since the example described concerns covering surfaces of micro-poreswith hydrophobic molecules the application of such modified micro-porouslayers for detecting substances with non-polar molecules is notrestricted to a gaseous ambiance. It is also feasible to detectnon-polar hydrocarbon dissolved in water, as far as there exists acondensable phase.

It is a feature of the invention that, by depositing a thin layer on thepore surface, the latter is provided with a property which decisivelydetermines the capability to attach matter depending on the polar ornon-polar character of the molecule, that is, of the wettability. Thus,for example, it is feasible to replace the methyl-groups used in theabove example by other groups, preferably bound to silicon, such asphenyl groups or amino groups to modify the selectivity of the sensorswith respect to certain hydrocarbons, amines and others. A phenyl group,for example, considerably increases the desired hydrophobic effect andis still stable up to temperatures of about 300° C. Deposits of, forexample, molecule groups such as C₃ H₇ NH₂ on the pore surfacesobviously result in a preferred affinity to substances from the class ofamines. Furthermore such surface deposits have the additional advantageof not requiring special filter membranes, which are otherwise requiredby comparable sensor elements of the previous art. In a secondembodiment, the layer of variable optical thickness is substituted by aporous dielectric organic layer which originally has a non-polar surfacecharacter. It is feasible to make such a porous polymer layer out of amixture of polymerizable monomers and an inert organic solvent. Themixture is deposited on a substrate. A porous layer is produced bypolymerization and subsequent removal of the inert solvent. Due to theiroutstanding optical transmission properties, methyl-methacrylate (MMA)or tri-ethylene-glycolic-di-methacrylate (TGMA) are utilized as amonomeric reactant, whereas octane is used as an inert solvent. Benzeneperoxide can be used as a polymerization initiator. Subsequent to acomplete polymerization and after removal of the inert solvent octane bywashing with acetone, a porous polymeric layer is obtained which has thedesired non-polar and hydrophobic surface, so that vaporous substanceswith non-polar molecules may condense in pores of the corresponding sizedepending on the vapor pressure. It is feasible to deliberately controlthe size and number of pores via the ratio of the mixture and thepolymerization conditions. The thickness of the layer may be preselectedto obtain optical effects desired by a particular application. To thisend, it is additionally feasible to combine the layer with other ones tolayer systems.

When more pores are required they can, of course, be produced by"irradiation processes", as common practice in microlithography, (forexample, by electron irradiation) and subsequent developing. PMMA-layersconventionally used in electron beam lithography are particularlysuited.

In a third embodiment, which can be optionally used in the applicationsdisclosed hereinabove, the layer 31 of variable optical thickness issubstituted by a dielectric layer which has no directly recognizablepore structure. However, it is capable to solve and, hence, absorb, forexample, non-polar hydrocarbon compounds or such having small moleculardipole moments, which may be smaller than the dipole moment of water. Inconsequence thereof, variations of the optical refractive number and/orthe thickness of the layer 31 result. However, water is not soluble inthese layers in concentrations worth mentioning. The layer or the layersare required to be capable of integration in an optical filter-layersystem or they are required to permit the setup of an opticalfilter-layer system. Typically, such layers consist of siloxanes, forexample, di-methylpolysiloxane (DMPS) and of polytetrafluorethylene (forexample, Teflon AF). In both substances, hydrocarbon compounds,particularly chlorinated hydrocarbons, are easily soluble, but water ishardly soluble. DMPS, having refractive numbers of 1.4 and slightlymore, may take the function of an optically high refracting layer andTeflon AF, having a refractive number of about 1.3, which is the valueof an optically low refracting layer so that an optical filter layersystem can be set up by an alternating sequence of such layers, withadjustable optical thickness which corresponds to a quarter and to ahalf, respectively, of an optical working wavelength. When hydrocarbonmolecules are absorbed by the layers, the optical effective thicknessvaries due to the refractive number and thickness variations dependentthereof, so that displacements of the optical working wavelength of thefilter result, at which point maximum transmission or reflection takesplace. It is feasible to evaluate the spectral displacements byrespective measuring techniques-with a high precision and reliability.

The features disclosed in the specification, the subsequent claims, andthe drawings are essentially for the invention individually, as well asin any combination.

We claim:
 1. An optical sensor element for detecting an organiccompound, comprising a spectral band filter having three layer systemsincluding two reflecting interference layer systems acting as reflectinglayers and an intermediate spacing layer consisting of a dielectricinorganic material having pores, the surfaces of the pores having amonomolecular coating of an organosilicon compound, said two reflectinginterference layer systems being on opposite sides of said spacinglayer, said spacing layer being an interference layer system of at leastone layer having an optical thickness of λ/4 or λ/2 or an integralmultiple of λ/4 or λ/2, wherein λ is a mean working wavelength, and saidmonomolecular coating providing a group with an affinity for the organiccompound to be detected, the optical thickness of said at least onelayer of the spacing layer being changed by the formation in said poresof liquid condensate of the compound to be detected.
 2. The opticalsensor element of claim 1, wherein said spacing layer has at least twolayers having alternating optical thicknesses of λ/4 or λ/2 or integralmultiples thereof.
 3. The optical sensor element of claim 1 or 2,wherein the dielectric inorganic material is a metal oxide which ishydrated at least at said surfaces of said cores and said coatingconsists of the product of reacting said hydrated metal oxide with anorganosilane compound of the general chemical formula R_(z)SiX.sub.(4-z) with 1≦z≦4, wherein X is a hydrolysable group and R is anon-hydrolysable organic group.
 4. The optical sensor element of claim3, wherein said hydrolysable group is selected from the group consistingof an alkoxy group and a halogen.
 5. The optical sensor element of claim1, wherein said organic compound to be detected is a hydrocarbon.